2001-2002 OTRC PROJECT: Deepwater Mooring & Riser Analysis
OBJECTIVE:
Springs are often employed in model tests of moored (especially deep-water) floating structure in wave basins. Existing numerical methods assume mooring lines to be inextensible or of small elongation. The assumption restricts their application to mooring lines with inserted large elongation elements, such as spring cells. The proposed research eliminates this restriction and makes the computation of mooring-line dynamics more accurate. The corresponding numerical codes will be applied to:
1) the design of truncated deep-water mooring-line model; and
2) the integration with the existing numerical schemes for computing wave loads on offshore structures to quantify the low-frequency damping induced by mooring lines.
APPROACH:
The newly developed numerical scheme can be applied to the simulation of small elongation elements with prescribed bending stiffness, such as risers and steel chains, as well as long elongation elements with negligible bending stiffness, such as spring cells. For studying the responses of moored deep-water offshore structures, a coupled analysis is required, which links the numerical scheme for dynamic mooring line/riser simulation with that for the prediction of hydrodynamic loading on a floating structure. Hence, the newly developed mooring line/riser numerical scheme will be integrated with 1) the numerical schemes based on wave diffraction theory, such as WAMIT and 2) the numerical schemes based on nonlinear wave kinematics (HWM) and the Morrison equation. The numerical results based on our numerical scheme will be examined against the corresponding experimental results, such as the measurements of mini TLP and FPSO recorded in the OTRC wave basin.
DEPLOYMENT OF RESULTS:
The numerical scheme for simulating mooring line/riser has many applications, which are crucial to the development of deepwater offshore structures.
1) It can be used to design truncated deep-water mooring lines to be tested in the OTRC wave basin. The static and dynamic characteristics of truncated mooring lines may better match those of undistorted mooring lines, not only statically but also in the magnitude of low-frequency damping.
2) Together with other numerical schemes for computing responses of a floating offshore structure, it can be used to simulate the responses of a moored deep-water offshore structure. Hence, the simulation can be used to examine and compare with the model tests conducted in the OTRC wave basin.
ANTICIPATED PROJECT DURATION: 1 year
PROJECT PLAN FOR YEAR 1 (2001-2002):
Scope of Work: Integrating the numerical scheme with other schemes such as WAMIT for simulating the responses of a moored floating offshore structure. Comparing the simulated results with corresponding wave basin measurements. The integrated numerical code will be delivered to the OTRC wave basin.
Sponsorship: OTRC Industry Sponsors
PRINCIPAL INVESTIGATOR (S) & OTHERS INVOLVED IN PROJECT:
PI(s): Jun Zhang
Others: Ms. X. Chen (PhD candidate), Mr. Y. Ding (Research Assistant)
OTRC PROJECT STATUS REPORT
Date: June 15, 2002
Project Name: Deep Water Mooring & Riser AnalysisProject Number: 32518/1510C (industry sponsored)
Principal Investigators: Jun Zhang
Estimated Completion Date: August 2002
Project Description:
1) A robust and efficient numerical scheme (COUPLE) is being continuously developed for simulating interactions between a floating offshore structure and its mooring-line/tendon/riser system. COUPLE consists of two major computation codes: a) CABLE3D for simulating the dynamics of a slender rod, such as a mooring line, tendon and riser, and b) two choices for computing wave/current/wind loads on the floating structure, based respectively on Morison equation/HWM and the combination of nonlinear wave diffraction theory (WAMIT) and Morison equation/HWM.
2) Improvements will be made a) to compute VIV of a floating structure (such SPAR, TLP etc) due to ocean currents, and b) to develop more efficient numerical scheme for simulating mooring-line/tendon/risers, which may significantly reduce the CPU time for the time-domain simulation of the motions of a floating structure and the tensions in its mooring-line/tendon/riser system.
Progress:
A primitive version of COUPE has been developed. It has been applied to the simulation of (six-degree-freedom) motions of a SPAR and TLP and tensions in their mooring lines/tendons/risers. The numerical simulations were compared with the corresponding laboratory measurement and satisfactory agreement between them is reached.
Reports & Publications:
Chen, X., Zhang J. & W. Ma (2001) ‘’Study on the Coupling Effects Between a Spar of and its Mooring System’’, Ocean Engineering, Vol.28, No.7 863-888.
Chen, X., Zhang J., Johnson, P., and Irani, M. (2000) ‘’Studies on the Dynamics of A Truncated Mooring System’’, Submitted to J. Offshore & Ploar Engrg..
Chen, X., Zhang J., Johnson, P., and Irani, M. (2001) ‘’Dynamic Analysis of Mooring Lines with Inserted Springs’’, Applied Ocean Res..Vol. 23, 277-284.
Chen, X., zhang, J., Johnson, P., and Irani, M. (2000) “Studies on Dynamics of Truncated Mooring Lines,” Proc. 10th International offshore and Polar Engineering conference, Vol. II, 94-101.
Chen, X. and Zhang, J. (2001) ‘’ Numerical Simulations of Truncated Mooring Lines with Inserted Springs ,” Proc. 11th International offshore and Polar Engineering conference, VI, Stravanger, Norway, pp. 635-642.
Chen, X., Zhang, J. Liagre, P., Niedzwecki, J. and Teigen, P. (2002)” Coupled Dynamic Analysis of A Mini TLP: Comparison with Measurements,” Proc. 21th International Conference on offshore Mechanics and Arcticr Engineering.
OTRC PROJECT STATUS REPORT
Date: Dec. 4, 2001.
Project Name: Deep Water Mooring & Riser AnalysisProject Number: 32518/1510C (industry sponsored)
Principal Investigators: Jun Zhang
Estimated Completion Date: August 2002
Project Description:
This project is to develop a coupled analysis for a moored floating structure. It links the numerical scheme for dynamic mooring line/riser simulation with that for the prediction of hydrodynamic loading on a floating structure. The mooring line/riser numerical scheme developed recently will be integrated with 1) the numerical scheme based on wave diffraction theory, namely, WAMIT and 2) the numerical schemes based on nonlinear wave kinematics (HWM) and the Morrison equation. The numerical results based on our numerical scheme will be examined against the corresponding experimental results, such as the measurements of mini TLP and FPSO recorded in the OTRC wave basin.
Progress:
1) Having completed the numerical scheme for simulating mooring line/riser with large elongation components.
2) Making predictions of the dynamics of mooring lines based on the newly developed numerical scheme. They are in satisfactory agreement with laboratory measurements and the results given by a commercial code Orcaflex. The new scheme is much more efficient than the commercial code.
2) Beginning the integration of the mooring-line numerical scheme with other schemes such as WAMIT for simulating the responses of a moored floating offshore structure.
Reports & Publications:
Chen, X., Zhang J. and Ma, W. (2001) ‘’Study on the Coupling Effects Between a Spar of and its Mooring System’’, Ocean Engineering, Vol.28, No.7 863-888.
Chen, X., Zhang J., Johnson, P., and Irani, M. (2001) ‘’Dynamic Analysis of Mooring Lines with Inserted Springs’’, Applied Ocean Res. (in press).
Chen, X., zhang, J., Johnson, P., and Irani, M. (2001) “Dynamic Analysis of Mooring Lines by Using Three Different Methods,” Proc. 11th International offshore and Polar Engineering conference, Vol. VI, 635-642.